1. Statement of the Technical Field
The invention concerns antenna systems, and more particularly pseudo-parabolic ring focus antennas configured for multi-band operation.
2. Description of the Related Art
It is desirable for microwave satellite communication antennas to have the ability to operate on multiple frequency bands. However, where space limitations constrain the size of the reflector dish, special techniques must be used to maintain antenna efficiency. One such technique is described in U.S. Pat. No. 6,211,834 B1 to Durham et al. (hereinafter Durham et al.), which concerns a multi-band shaped ring focus antenna.
In Durham et al., a pair of interchangeable, diversely shaped close proximity-coupled sub-reflector-feed pairs are used for operation at respectively different spectral frequency bands. Swapping out the subreflector/feed pairs changes the operational band of the antenna. Advantage is gained by placement of the shaped sub-reflector in close proximity to the feed horn. This reduces the necessary diameter of the main shaped reflector relative to a conventional dual reflector antenna of the conventional Cassegrain or Gregorian variety. The foregoing arrangement of the feed horn in close proximity to the sub-reflector is referred to as a coupled configuration.
The coupled configuration described in Durham et al. generally involves sub-reflector to feed horn spacing on the order of 2 wavelengths or less. This is in marked contrast to the more conventional sub-reflector to feed horn spacing used in a decoupled configuration that is typically on the order of several to tens of wavelengths. Notably, use of a coupled configuration also obviates the problem of phase center migration with frequency as may occur with conventional sub-reflector designs that utilize a decoupled configuration.
One problem with systems that utilize such ring focus reflector geometries is that there is a fundamental limit on the electrical size of the sub-reflector for each feed/subreflector configuration. In the coupled configuration described in Durham et al., the electrical size of the sub-reflector cannot be too large or the feed system for the sub-reflector will fail. In fact, the failure of the feed system resulting from an excessively electrically large sub-reflector is generally the limiting factor in determining the highest operating frequency of an antenna system as described in Durham et al. By comparison, in conventional dual reflector Cassegrain and Gregorian type reflector systems using feed horns and sub-reflectors arranged in accordance with a decoupled configuration, the electrical size of the sub-reflector cannot be too small or the system optics will fail. However, the conventional Cassegrain and Gregorian type reflector systems will not operate with a sub-reflector/feed arranged in a coupled configuration.
From the foregoing it may be appreciated that limitations on sub-reflector size in the various types of antennas and other factors relating to performance have generally created a practical limit to the range of frequencies over which a particular antenna system will operate effectively. Accordingly, new techniques are needed to expand the useful operating range of frequencies to permit dual reflector microwave antenna systems to operate effectively given size and performance constraints on four or more spectrally offset frequency bands.
The invention concerns a ring focus antenna and method of using same. The ring focus antenna can have a main reflector of revolution shaped as a non-regular paraboloid about a boresight axis of the antenna. A sub-reflector/feed pair is provided comprising a sub-reflector of revolution shaped as a non-regular ellipsoid having a ring-shaped focal point about the boresight axis. A feed element is installed at a feed element location separated spaced from a vertex of the sub-reflector on the boresight axis of the antenna. The main reflector is adapted for operation with a sub-reflector/feed pair having a coupled configuration and a sub-reflector/feed pair having a decoupled configuration. The main reflector is operable at a plurality of spectrally offset frequency bands. For example, the antenna can be designed for operation over C-band, X-band, Ku-band, and Ka-band.
A coupled configuration one of the sub-reflector/feed pairs is advantageously installed on the main reflector for operation of the antenna at a lowest one of the frequency bands. The feed element can further include a feed aperture that is spaced from the vertex of the sub-reflector. The spacing is generally less than about 2 wavelengths for the coupled configuration. A feed aperture can be spaced from the vertex by more than about 5 wavelengths for the decoupled configuration.